That doesn't make legal sense in actual fact. Nobody would be linking to such sites if it were indeedactionable. It's embedding, and is only actionable IF the original material posted to YT is not owned by the poster (films, TV shows, and the like)and linking to a site where the material is licenced is 100 percent legal. It's simply Vincent protecting the forum - which is his job so to do, even if he doesn't understand the copyright laws
It will make PF more boring of course - or does, because Polish music is part of the culture, and no action can come through embedding single music videos here - if they broke DMCA rules, they would be taken down from YT anyway.
All completely needless, but like I say, it's Vincent playing it safe.It could be also because he's concerned about Google Adsense action. He shouldn't be, but it is what it is.